This invention relates to a balancer for use in a centrifugal rotary machine, and more particularly, to a balancer for use in a centrifugal rotary machine which is designed to reduce the abnormal vibration of the support means for supporting a cylindrical rotary tub, which occurs due to the unbalanced distribution of an article in the rotary tub.
For example, in an electric washing machine with a dehydrating function, the cylindrical rotary tub which houses washing is often abnormally vibrated during the dehydrating operation due to the unbalanced distribution of the washing. The suspended tub which supports as support means the rotary tub is abnormally vibrated by the vibration of the rotary tub. Generally, such a rotary tub is equipped with a balancer in order to prevent this abnormal vibration. Examples of this type of balancer are a solid balancer, a liquid balancer, and a ball balancer.
The solid balancer is a ring-like weight which is coaxially disposed on the inner circumferential surface of the rotary tub. This solid balancer is designed to increase the moment of inertia of the rotary tub, thereby reducing the amplitude of the rotary tub caused by an unbalanced load.
The liquid balancer uses liquid as a balancing weight, said liquid being flowably sealed into a cylindrical casing coaxially equipped to the rotary tub. When the rotary tub is rotated at a rotating rate higher than that corresponding to the resonance rotating rate, the liquid moves toward the side opposite to that on which the washing is loaded, i.e., that on which the unbalancing load is located, thereby absorbing the unbalance of the rotary tub. Thus, the vibration of the suspended tub is decreased. Although the liquid balancer can cope with a relatively wide variety of weights of unbalanced loads, it has the drawback of failing to zero the vibration amplitude of the rotary tub.
The ball balancer uses a steel-made ball, or spherical body or bodies, as a balancing weight, said balls being circumferentially and movably disposed in an annular casing, as in the liquid balancer. The ball has the characteristic that when the rotary tub is rotated at a higher rotating rate than that corresponding to the resonance rotating rate, the ball is moved to the side opposite to that on which the unbalancing load is located, thereby zoning the amplitude of the vibration of the rotary tub. On the other hand, however, the ball has the drawback that when the rotary tub is rotated at a lower rotating rate than that corresponding to the resonance rotating rate, the ball does not rotate integrally with the rotary tub but moves freely throughout the interior of the casing independently of the location of the unbalancing load in the rotary tub, thereby increasing the tub's unbalance. In order to rectify this drawback, a ball balancer has heretofore been proposed in which the balls are sealed into the casing together with a small amount of highly viscous liquid. The balls come to rotate integrally with the rotary tub in a short period of time due to the viscosity of the liquid, thus eliminating the disadvantage mentioned above. That type of ball balancer, however, has the shortcoming that while the rotary tub is making a normal rotation, it becomes impossible due to the viscosity of such liquid for each ball to move smoothly to a theoretical position. For this reason, the amount of movement of the balls in such a ball balancer depends on chance or uncertain conditions, resulting in a variation of the vibration-reducing effect of the balls with respect to the same weight of unbalanced load.
The respective characteristics of the said balancers will now be compared with reference to an electric washing machine 10 with a dehydrating function such as, for example, that shown in FIG. 1. The washing machine 10 will first be described. The washing machine 10 comprises an outer case 12, a suspended tub 16 as support means elastically suspended within the outer case 12 by means of suspension bars, and a rotary tub 18 rotatably supported in the suspended tub and used for washing, rinse and dehydration. In the rotary tub 18 a stirring vane 20 is disposed. The rotary tub 18 and the stirring vane 20 are selectively driven by a motor through a power transmission mechanism 22. A balancer 26 is equipped to the upper end portion of the inner circumferential portion of the rotary tub 18. FIG. 2 is a linear diagram showing the respective characteristics of the solid balancer, liquid balancer and ball balancer in relation to the said washing machine 10. In FIG 2, Q represents the weight of unbalanced load in the rotary tub resulting from the biased location of a washing in the rotary tub 18, while A represents the vibration amplitude of the suspended tub 16. In FIG. 2, numeral 28 indicates a characteristic line of the solid balancer when the same was used as the balancer 26. Numeral 30 indicates a characteristic line of the liquid balancer when used as the balancer 26. Numerals 32, 33 indicate characteristic lines of the ball balancer when used as the balancer 26. The characteristic line 32 indicates a case in which the rotary tub 18 is formed of a material difficult to bend, such as metal, while the characteristic line 33 is used when the rotary tub is formed of a material likely to bend, such as plastic. The characteristic lines 28, 30 and 32 will now be compared with each other under the condition in which the rotary tub is formed into an ideal structure which is not bent whatsoever. As shown by the characteristic line 32, the ball balancer has the best vibration-reducing performance, and the liquid balancer has the second best vibration reducing performance. However, because the rotary tub 18 is generally formed of plastic material of high productivity, the vibration-reducing performance of the ball balancer is actually that shown by the characteristic line 33. This means that the performance of a ball balancer is lower than that of the liquid balancer.
Hereinafter, the technical phenomena occurring when the rotary tub 18 is bent or flexed will be explained with reference to FIGS. 3 to 5. In FIG. 3, there are schematically shown the rotary tub 18 housing a washing 34, constituting an unbalanced load, and a ball balancer 38 having a plurality of balls or spherical bodies 36 and equipped to the rotary tub 18. In FIGS. 4 and 5, the ball balancer 38 and the rotary tub 18 are schematically shown to include the suspended tub 16 and a drive shaft 40. FIG. 3 shows the state in which the washing 34 is distributed in an unbalanced manner when the rotary tub 18 is rotated at a higher rate than that corresponding to the resonance rotating rate or at a rate corresponding to normal rotating rate. A centrifugal force f.sub.b produced around the rotational center axis (line P.sub.1 -P.sub.2) acts on each spherical body 36, and component forces f.sub.1 and f.sub.2 of the centrifugal force f.sub.b are existent. Because of this resultant force f.sub.2, each spherical body 36 is circumferentially moved to the side opposite to that on which the unbalanced load 34 is located, namely, to the lower-load side of the rotary tub 18. Each spherical body 36 is moved to the said oppositeside of the rotary tub 18 until the rotational center point P.sub.2 is brought to coincide with the rotational center point S.sub.3 of the balancer 38, i.e., until the vibration amplitude of the rotary tub 18 is zeroed. The ball balancer 38 thereby absorbs the unbalance of the rotary tub 18 and thus acts to reduce the vibration of the rotary tub.
Where the rotary tub 18 is formed of metal, the marginal unbalance-absorption point of the ball balancer 38 is located at the position R.sub.1 of the characteristic line 32 shown in FIG. 2. At that marginal unbalance-absorption point, the spherical bodies 36 of the ball balancer 38, as shown in FIG. 3, continue to abut on each other. In other words, they remain gathered together. When the weight of the unbalanced load in the rotary tub 18 is increased and exceeds a value corresponding to the said marginal unbalance-absorption point, the ball balancer 38 becomes unable to absorb the unbalance of the rotary tub. As a result, as shown by the line portion of the characteristic line 32 extending from the point R.sub.1 in the rightward direction of the illustration, the variation amplitude of the suspended tub 16 increases sharply.
Further, where the rotary tub 18 is formed of plastic material, the rotary tub is bent as shown in FIG. 4. When each spherical body 36 is moved circumferentially around the rotary tub 18 to the opposite side thereof where the unbalanced load 34 is located, the rotary tub is shifted until the rotational center point P.sub.2 is brought to coincide with the rotational center point S.sub.3 of the balancer 38. That shift is due to a resultant force F.sub.b of the respective centrifugal forces of the spherical bodies 36. However, where the rotary tub 18 is formed of plastic material, the rotary tub, upon receipt of such resultant force F.sub.b, is shifted as above only at the upper end portion. That is, the rotary tub 18 is bent so that the position of its upper end portion may be shifted by .DELTA.a relative to the center axis (line S.sub.1 -S.sub.2) of the rotary tub. As a result, the rotational center axis (line P.sub.1 -P.sub.2) is deviated by the amount of flexure .DELTA.a from the center axis (line S.sub.1 - S.sub.2) of the rotary tub 18. Accordingly, though the vibration amplitude of the rotary tub 18 is zeroed, that of the suspended rub 16 fails to be zeroed due to the existance of said .DELTA.a deviation. The vibration-reducing characteristic of the ball balancer is thus as indicated by the characteristic line 33 of FIG. 2. When the unbalanced load is a larger one, each spherical body 36 is further moved to the side opposite to that on which the unbalanced load 34 is located, and the resultant force F.sub.b acting on the rotary tub 18 increases. As a result, the amount of flexure .DELTA.a of the rotary tub 18 further increases, so that the vibration-reducing action of the ball balancer 38 decreases even more. As shown by the point R.sub.2 of FIG. 2, at the marginal unbalance-absorption point of the ball balancer 38 at which the spherical bodies 36 are kept together, the resultant force F.sub.b of the centrifugal forces acting on the spherical bodies reaches it maximum. Thus, the amount of flexure also reaches its maximum. Thereafter, even when the weight of the unbalanced load is increased to exceed a value corresponding to the marginal unbalance-absorption point R.sub.2, said resultant force F.sub.b does not increase and the amount of flexure .DELTA.a of the rotary tub 18 is kept constant. The amplitude of the vibration of the rotary tub 18 occurring due to an unbalanced load greater in weight than the unbalanced load corresponding to the marginal unbalance-absorption point R.sub. 2 differs 180.degree. in phase from the amount of flexure .DELTA.a of the rotary tub. Therefore, after the marginal point R.sub.2 is reached, the center point of the suspended tub 16, with a further increase in the weight of the unbalanced load therein, gradually approaches the rotational center point P.sub.2. As indicated by the point R.sub.3 in FIG. 2, the vibration amplitude of the suspended tub 16 is eventually zeroed. Thereafter, when the weight of unbalanced load is further increased, the positional relationship of S.sub.2 with P.sub.2 is reversed, as shown in FIG. 5. As indicated by the point R.sub.3 and the succeeding line portion of the characteristic line 33 of FIG. 2, the vibration amplitude of the suspended tub 16 is once again increased.
As stated above, the ball balancer has the best vibration-reducing action. If the rotary tub is formed of plastic material, its vibration amplitude is reduced due to its flexure nearly to zero after the marginal unbalance-absorption point of the ball balancer is reached, then it is once again increased. However, in this case, the vibration amplitude of the suspended tub is rapidly decreased and, with only a slight additional increase in the weight of unbalance load, is once again increased. For this reason, as indicated in the characteristic line 33 of FIG. 2, in the region where the weight of unbalanced load is relatively great, the vibration-reducing action or unbalance-absorbing action of the ball balancer varies greatly relative to the slight variation of the weight of unbalanced load. Accordingly, the ball balancer with this unbalance-absorbing action fails to give a stable vibration and noise reducing effect when put to practical use.
There has also been proposed an electric washing machine with a dehydrating function, the rotary tub of which is equipped with a plurality of ball balancers and liquid balancers (Japanese Utility Model Laid-Open No. 51680/76). This type of washing machine can be expected to have an excellent vibration-reducing effect by combining the respective characteristics of the liquid balancer and ball balancer. However, it has the drawback that the rotary tub is complicated in structure, thus increasing the manufacturing cost. Further, because the outer circumferential surface of the rotary tub is surrounded by the balancers, the rotary tub can not be formed with a large number of dehydrating openings. As a result, the dehydrating performance of the washing machine is degraded. Further, no measures are taken to prevent the ball and liquid balancers from making unruly movements at the starting time of the rotary tub. Thus, such a washing machine has the drawbacks of noise generation due to the collision of one spherical body against another and an increase in the vibration of the rotary tub.